Spectroelectrochemistry (ch. 17)

1. UV & visible spectroscopy (1) Transmission experiments (2) Ellipsometry

(3) Internal reflection spectroelectrochemistry: surface plasmon resonance (4) Second harmonic spectroscopy

2. Vibrational spectroscopy:

(1) IR spectroscopy

(2) Raman spectroscopy

3. Electron & ion spectroscopy

XPS, AES, LEED, HREELS, mass spectroscopy 4. Magnetic resonance methods: ESR, NMR 5. Quartz crystal microbalance

6. X-ray methods: XAS, XRD

A

Spectroelectrochemistry (ch. 17): in situ & ex situ

Fall, 2012

Electrochemical Energy Engineering

Electromagnetic radiation

Electric component of electromagnetic wave

Velocity of propagation v_i

Frequency ν: number of oscillations per second

In a vacuum, v_i is independent of wavelength and a maximum

→ c = 2.99792 x 10⁸ m/s

In a air, v_i differs only slightly from c (about 0.03% less): ~ c

Wavenumber ν: the reciprocal of wavelength in cm (cm^-1)

eV nm cm

^-1

Hz

The electromagnetic spectrum

Emission

Energy states of chemical species

Energy states:

Electronic states Vibrational states Rotational states

Ground state and excited states

chemiluminescence

Absorption

Luminescence

Photoluminescence (fluorescence (형광) and

phosphorescence(인광))

Beer’s law

Quantitative aspects of spectrochemical measurements

Transmittance T Absorbance A

Beer’s law A = abc, where a: absorptivity (Lg^-1cm^-1), b: path length through the medium (cm), c: concentration of absorbing species (gL^-1)

Or, A= εbc, where ε: molar absorptivity(Lmol^-1cm^-1), b(cm), c(mol/L)

1. UV & visible spectroscopy

(1) Transmission experiments

Simplest spectroelectrochemical experiment

Absorbance change

Optically transparent electrode (OTE): ITO, Au or Pt on glass, minigrids

Cells for transmission spectroelectrochemistry

Electrochemical cuvette

Responses for transmission spectroelectrochemistry: absorbance vs. time

Oxidation of o-tolidine

Spectra of cobalt complex at different potentials

-0.90 V to -.1450 V from a to j

Co(II) at -0.9 V and Co(I) at -1.45 V

Coulometric titration (reduction) of cytochrome c and oxidase

By methyl viologem (MV²⁺)

Detector Power meter

Potentiostat

Electrochemical cell or EC devices

He-Ne laser (633nm)

In-situ transmittance test

(2) Ellipsometry

Change of electrode surface → change of reflecting properties

Polarization (편광)

Polarization of radiation ( 편광 )

Ordinary radiation consists of a bundle of electromagnetic waves in which the vibrations are equally distributed among a huge number of planes centered along the path of the beam

Transmission of radiation

Radiation through a transparent substance

Refractive index (굴절율) of a medium is one measure of its interaction with radiation

The velocity of the radiation in the medium (v_i) Most liquid: n_i = 1.3~1.8

Solid: 1.3~2.5+

Interaction → polarization(분극, temporarily deformation (10^-14~10^-15 s) of atom & molecular species in medium

Interaction → wavelength change → variation of n_i with wavelength,

“dispersion”(분산)

Radiation passes at an angle through the interface between two transparent

media that have different densities → refraction as a consequence of a difference in velocity of the radiation in two media.

Refraction of radiation ( 굴절 )

Snell’s Law

If M₁ is vacuum, v₁ = c, n₁ = 1

Radiation passes at an angle through the interface between two transparent media that have different densities → reflection always occurs

Fraction of reflection

Reflection of radiation ( 반사 )

I₀: intensity of the incident beam, I_r: the reflected intensity

Reflection

Four types:

Specular reflection: smooth surface Diffuse reflection

Internal reflection

Attenuated total reflection (ATR)

Specular reflection (

정반사

)

Internal reflection (blue line) Diffuse reflection

Reflection of polarized light from a surface

Parallel to incident plane: p Perpendicular: s

Other angle: resolve p and s

If a linearly polarized beam is reflected from a surface, the parallel and perpendicular components undergo different changes in amplitude and phase

Pairs of rays → elliptically polarized (circular polarization: equal amplitides, 90° phase shift

Elliptic polarization arising from a phase shift between parallel & perpendicular components

Elliptical polarization:

In electrodynamics, elliptical polarization is the polarization of electromagnetic radiation such that the tip of the electric field vector describes an ellipse in any fixed plane

intersecting, and normal to, the direction of propagation. An elliptically polarized wave may be resolved into two linearly polarized waves in phase quadrature, with their

polarization planes at right angles to each other.

Since the electric field can rotate clockwise or counterclockwise as it propagates, elliptically polarized waves exhibit chirality.

Other forms of polarization, such as circular and linear polarization, can be considered to be special cases of elliptical polarization.

(from Wikipedia)

Ellipsometry

Change of electrode surface → change of reflecting properties -Difference in phase angle: ∆

-Ratio of electric field amplitudes Ψ

Ellipsometric results for anodization of Al in tartaric acid

Growth of passive film on Fe at 0.8 V vs. SCE

Growth of polyaniline film: experiemental (dotted) vs. fiited resluts (solid)

(3) Internal reflection spectroelectrochemistry

Cell assembly Internal reflection → light absorption by species

at the interface

Internal reflection (blue line)

Surface plasmon resonance (SPR)

Surface plasmon resonance (SPR) is the collective oscillation of valence electron in a solid stimulated by incident light. The resonance condition is established when the frequency of light photons matches the natural frequency of surface electrons

oscillating against the restoring force of positive nuclei. SPR in nanometer-sized structures is called localized surface plasmon resonance. SPR is the basis of many standard tools for measuring adsorption of material onto planar metal (typically gold and silver) surfaces or onto the surface of metal nanoparticles. It is the fundamental principle behind many color-based biosensor applications and different lab-on-a-chip sensors. (from Wikipedia)

Surface plasmon resonance (SPR)

플라스몬(plasmon)이란 금속 내의 자유전자가 집단적으로 진동하는 유사 입자를 말한 다. 금속의 나노 입자에서는 플라스몬이 표면에 국부적으로 존재하기 때문에 표면 플 라스몬(surface plasmon)이라 부르기도 한다. 그 중에서도 금속 나노 입자에서는 가시~

근적외선 대역 빛의 전기장과 플라스몬이 짝지어지면서 광흡수가 일어나 선명한 색을 띠게 된다. (이 경우, 플라스몬과 광자가 결합되어 생성하는 또다른 유사 입자를 플라 스마 폴라리톤이라고 한다.) 이 현상을 표면 플라스몬 공명(surface plasmon resonance) 이라 하며, 국소적으로 매우 증가된 전기장을 발생시킨다. (위키백과)

SPR curves for Au and adsorbed monolayers

The shift of SPR minimum → change in interface: determine the

thickness of the adsorbed layer

(4) Second harmonic spectroscopy

Second harmonic generation (SHG): ω → 2ω

Second harmonic generation (SHG): noncentrosymmetric crystals If symmetry is broken at the solid/liquid interface → SHG signal

SHG signal is sensitive to species at the interface: used to detect adsorbed species, reaction intermediates etc

Polarization (순간적 찌그러듬), P = αE α: proportionality constant, E: electric field

At high radiation intensities (lasers) → nonlinear optical effect P = αE + βE² + γE³ +..

where α>β>γ

약한 강도의 빛: 첫번째 항만 중요 (직선관계) 강한 강도의 빛 (레이저): 두번째, 세번째 항 중요

두 항만 고려하면, P = αE_msinωt + βE_m² sin²ωt sin²ωt = (1 – cos2ωt)/2를 이용하면,

P = αE_msinωt + [βE_m² /2](1 – cos2ωt) 첫 항: 선형관계 (linear) (at low radiation intensities)

두번째 항: 비선형(non-linear), frequency가 2ω, 즉 입사 주파수의 두배(double)가 됨

→ “frequency-doubling process”: 단파장 레이저 만드는데 많이 씀

Nd-YAG: 1064 nm IR → 532 nm green (30% yield, potassium dihydrogen

phosphate(dielectric 물질)에 통과) → 266 nm UV (ammonium dihydrogen phosphate 통과)

Non-linear optical effects

SHG response

Polycrystalline Pt in HClO₄/KCl CV vs. SHG signal

Neg. potential: adsorbed hydrogen 0~0.4 V: adsorbed chloride ion

>0.4 V: oxide or adsorbed hydroxyl

Adsorption isotherm at 0.2 V at different KCl

concentration using SHG signal